Method for prestressing stranded cable



Nov. 5, 1957 B, J, s m 2,811,773

METHOD FOR PRESTRESSING STRANDED CABLE Filed July 29, 1954 v INVENT0R.V M/m/ /v J lam/my BY United States Patent METHGD FOR PRESTRESSING STRANDED CABLE Benjamin J. Baskin, Philadelphia, Pa., assignor, by mesne assignments, to American-Marietta Company, Chicago, 111., a corporation of Illinois Application July 29, 1954, Serial No. 446,466

2 (Jlairns. (Cl. 29-452) This invention relates to a novel method for prestressing stranded wire cable, and particularly to an improved method for prestressing stranded wire cable in the manufacture of prestressed concrete structural units.

During recent years the use of precast concrete structural units has been given recognition in the construction industry, with such units being used widely in the construction of such structures as small bridges and the like. These units are customarily precast at the factory, stripped from the casting forms, and transported to the work site Where they are then installed.

It has been found, and it is now well recognized, that when a concrete structural unit is subjected to prestress ing, so as to create compressive stresses within the concrete, per se, the structural characteristics of such units are greatly improved, especially when subjected to bending under conditions of beam loading. Thus a small bridge deck member which is prestressed is enabled to withstand a greater load than will a comparable deck member that is not so prestressed.

One of the practices which has been followed to date in prestressing concrete structural units is to include in these concrete units stranded cables or wires which are set up in the forms or molds in which the units are cast, and prestressed by applying tension forces thereto. Concrete is then poured into the forms about the cables while the said stranded cables are in a prestressed condition. Then after the concrete has firmly set about the cables, the cables are cut, whereupon the said cables in attempting to revert to their original unstressed condition create compressive stresses within the concrete, per se. It is these compressive stresses that tend to offset the tensile stresses of bending, thus improving the bending characteristics of concrete structural units of this type.

In prestressing the stranded cables in the manufacture of these prestressed concrete structural units, as described above, it has been the practice to fixedly anchor the various individual cables at one end of the mold and then apply a tensile force to the opposite free ends of the cables so as to set up a tensile stress therein. In as much as the various cables have been individually stressed, it has been found that an undesirable lack of uniformity has existed in the degree of stressing from cable to cable. I have accordingly described and claimed in my copending application, Serial No. 446,465 filed July 29, 1954, a screw jack and tension clamp assembly for use with the casting form in the manufacture of these prestressed concrete units, which assembly is capable of accommodating a plurality of stranded cables at one time and applying a uniform degree of tension to each and all of the cables held thereby. It is with the method of holding these stranded cables and applying a uniform degree of tension to each and all of said cables that the present invention is concerned.

t is an object of this invention, therefore, to provide an improved method for prestressing stranded cable in the manufacture of prestressed concrete structural units.

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It is another object of this invention to provide an improved method for prestressing stranded cable in the manufacture of prestressed concrete structural units wherein a plurality of cables are prestressed at the same time.

It is a further object of the invention to provide an improved method for prestressing stranded cable in the manufacture of prestressed concrete structural units wherein a uniform degree of tension is applied to a plurality of cables at one time.

These objects, together with other objects and advan tages to be derived from the present invention, will become apparent by reference to the following detailed description of the invention in conjunction with the accompanying drawings wherein similar numerals refer to similar parts throughout the several views.

Figure l is a side elevational view in schematic of a form or mold used in the manufacture of prestressed concrete structural units illustrating the first step of the present method for prestressing stranded cable.

Figure 2 is a view similar to that of Fig. 1, illustrating the step of locking the stranded cable in the jack block.

Figure 3 is a view similar to that of Fig. 2, illustrating a further cable-locking step in the method of the present invention.

Figure 4 is a view similar to that of Fig. 3, illustrating still another step of the persent method.

Figure 5 is a side elevational view in schematic, illus trating the final step of stressing the stranded cables in the mold, prior to the pouring of concrete therein.

Referring to the drawings, it will be seen that a form or mold used in the manufacture of prestressed concrete structural units is shown therein, in schematic, as consisting of bed 10, anchor blocks 11 and 12 at both ends of said bed, an anchor bar 13 fixedly attached to the anchor block 12, a jack block support bracket 14 fixedly attached to the anchor block 11, and a jack block 15 slidably supported on the support bracket 14, which jack block is adapted to move horizontally on said bracket outwardly from the anchor block 11 under the action of stressing screws 16 threadably supported therein and abutting against the anchor block 11. The anchor blocks 11. and 12 are provided with openings through which stranded cable may be threaded in attaching the cable to the mold, and the jack block 15 is provided with a plurality of tapered apertures 17 in alignment with the openings in the anchor blocks, in which apertures the ends of the cable are fixedly secured prior to stressing thereof in a manner to be described below.

Looking now at Fig. 1, a spool of stranded cable is referred to therein by the numeral 18. The first step of the present method is to pass the free end of the cable 18 on said spool through a collar 19, and then through a lower aperture 17 in the jack block 15 and the lower openings in the anchor blocks 11 and 12. The said cable is then bent around the anchor bar 13 and passed back through the upper openings in the anchor blocks 11 and 12 and through the upper tapered aperture 17 in the jack block in vertical alignment with the lower phase of the cable 18. It will be noted that the jack block 15 is, at this point, in abutting relationship with the anchor block 11.

Having thus initially threaded the stranded cable 18 through the anchor blocks 11 and 12 and the jack block 15, the next step is to securely anchor the ends of the said cable in the jack block so that the cable may be stressed. To accomplish this, a split wedge member 20 is inserted into the upper tapered aperture 17, as shown in Fig. 2, so as to surround the cable 18, the said wedge member being scored on its cable-contacting surface for firmly gripping said cable. A second collar 21 is then passed over the end of the cable and into the aperture 17 in contact with the wedge 20. A hammer bar 22,

having a grooved end to accommodate the cable 18, is then placed in abutting relationship with the collar 21 and a blow is applied thereto, as shown by the arrow in Fig. 2, so as to drive the wedge 20 deep into the aperture 17, thus fixedly securing the free end of the cable 18 in the jack block 15. The collar 21 and hammer bar 22 may then be removed.

With the one end of the cable 18 thus attached, the bottom phase of the said cable is pulled in the direction indicated by the arrow in Fig. 3 until all of the slack is out of the cable and the cable is taut. During this operation, a scale is attached to the cables to insure that the initial tautness thereof, prior to stressing, is the same in each cable. A second split wedge 23 is then inserted into the lower aperture 17 of the jack block 15 and the collar i9 is placed in abutting relationship therewith. The hammer bar 22 is next placed adjacent to the collar 19 and a blow applied thereto in the direction of the arrow shown therewith, thus driving the wedge 23 deep into the lower aperture 17 and locking the cable 18 in place therein. The hammer bar 22 and collar 19 are then removed and the cable 18'cut. With the cable thus securely anchored in the jack block 15 and maintained in a taut condition throughout the casting form the said cable is now ready to be prestressed.

To apply a tensile force to the stranded cable 18 for prestressing purposes, a stressing screw 16 is threadably inserted into the jack block 15 in a threaded bore prw vided therein, until the end of said screw contacts the face of the anchor block 11. Then as the stressing screw 16 is rotated, the jack block 15 travels outwardly from the anchor block 11 in sliding relationship with the jack support bracket 14, so as to exert a tensile force upon the cable 18 attached thereto. It will be noted that since all the slack has been eliminated from the upper and lower phases of the stranded cable, the said phases are stressed uniformly. In addition, since the jack block 15 is adapted to receive a plurality of lengths of cable 18, the various cables are not only prestressed uniformly, but also simultaneously. With the stranded cables thus prestressed, concrete may be poured into the mold, surrounding said cables, and allowed to set, and when the concrete has reached the desired compressive strength, the ends of the cables are cut prior to removal of the concrete unit from the mold.

It is thus seen from the above description of the present invention, that an improved method is presented for prestressing stranded cable in the manufacture of prestressed concrete structural units, wherein a plurality of such cables within a given structural unit are simultaneously stressed so as to be put under a uniform degree of tension.

Although the present discussion of this invention has been limited to the above-described preferred series of steps, variations thereof are possible without departing from the spirit of the invention. It is not intended, therefore, that the invention be limited to the preferred series of steps described, but only to the inventive concept as defined by the appended claims.

What is claimed is:

1. In the manufacture of prestressed concrete structural units, the method for prestressing stranded cable which comprises the steps of: (a) supporting a length of stranded cable in a casting form; ([1) folding the cable in U-shaped fashion with the U-shaped bight over an anchor bar which is fixed with respect to the cable end which is tensioned so as to lie in a common plane with the free ends thereof in spaced adjacent relationship; (0) anchoring one free end of the cable to a movable member; (d) anchoring the other end of the cable to said movable member; and (e) applying tension forces to both anchored cable ends by means of said movable member to place both cable ends in substantially parallel relation with each other, so as to cause prestressing of the cable. 2. In the manufacture of prestressed concrete structural units, the method for prestressing stranded cable which comprises the steps of: (a) supporting a plurality of stranded cables in a casting form in parallel horizontal relationship to one another; (b) folding the individual cables in U-shaped fashion over an anchor bar which is fixed with respect to the cable end which is tensioned so as to lie in a vertical plane with the ends of each cable lying in spaced adjacent relationship to each other; (c) anchoring the uppermost cable ends to a movable member; (d) anchoring the lower cable ends on said movable member; and (e) applying a tension force to said movable member so as to cause the simultaneous and uniform stressing of the cables.

References Cited in the file of this patent UNITED STATES PATENTS 2,609,586 Parry Sept. 9, 1952 2,637,895 Blaton May 12, 1953 2,696,040 Crom Dec. 7, 1954 

